1. Field of the Invention
The invention generally relates to transmission links for frequency division multiplex or time division multiplex signals equipped with repeaters-regenerators and more particularly to a supervisory system of the repeaters-regenerators inserted in a link section between a monitoring station and a monitored station for monitoring performance and identify faulty units; more precisely, the function of this supervisory system is to transmit information relative to the status of the repeaters-regenerators to a control point, normally the monitoring station.
It is well known to provide the repeaters-regenerators with error monitoring circuits. The error information collected in each repeater-regenerator is obtained by sampling the state of a number of alarm trigger circuits and by counting the number of in-line code errors and coding this number. The resulting binary pattern is stored in a register in the repeater-regenerator. For example, if the in-line code is the bipolar code, the error monitoring circuit counts the number of bipolarity viols. If the in-line code is the high density bipolar code, the error monitoring circuit counts the number of sequences containing more than n zeros.
The binary pattern in the register is then pulsed out of the register at the supervisory transmission rate and used to modulate a carrier. The resulting signal may then be demodulated at the line terminal and interpreted as required.
2. Description of the Prior Art
The article "A 120 Mb/s Digital Transmission System on Coaxial Cable", by J. P. WAKELING, published in POINT-TO-POINT COMMUNICATION, Vol. 18, Nr. 1, January 1974 discloses a supervisory system for a digital link in which a plurality of audio-frequency carriers, each of which is associated with one power fed repeater-regenerator, are modulated by the binary pattern identifying the faulty units. In this supervisory system, there is a need for as many carriers as repeaters-regenerators plus an error rate signal and at least two pass-band filters in each repeater-regenerator for separating the associated carrier and the error rate monitor control carrier.
U.S. Pat. No. 3,987,395 issued Oct. 19, 1976, discloses a further system of telemonitoring for digital links equipped with one-way repeaters-regenerators. According to this patent, each repeater-regenerator is provided with an error detector and counter having an output supplying an error rate coder, in turn connected to a supervisory register. The repeater-regenerator receives a time division multiplex frame with a framing word and comprises
means for retiming the clock pulse of the frame; PA1 means for counting the successive numbers of the time-slots; and PA1 means for entering the contents of the supervisory registory register into the time-slot having a number assigned to the repeater-regenerator.
Such a system is not adapted to monitor a link equipped with two-way repeaters-regenerators in which the supervisory information generated in a given repeater-regenerator is transmitted to the monitoring station from said given repeater-regenerator directly through the backward link. The time division multiplex frame thus loses a time-slot at each repeater-regenerator, causing the number of time-slots in the frame to decrease as the frame travels on the forward link.
Further, the control signal in the link must comprise both a low frequency signal and clock pulses defining the time division multiplex frame and the repeaters-regenerators must include clock pulse restoring means.
The object of the invention is to provide a monitoring system for telecommunication forward and backward links equipped with two-way repeaters-regenerators, wherein the system only needs a low frequency signal, frequency multiplexed with the data to be transmitted.